Among approaches for picking up a sound, a method for recording a sound with microphones attached to near both ears of a human head or a dummy head is generally called “binaural sound recording”. Simultaneously recording sound space information on a particular place where a sound is recorded is considered as a primary advantage of the binaural sound recording. When an audio signal obtained through the binaural sound recording is played back with a pair of headphones, an effect as if a sound is heard in an environment where the sound recording has been carried out can be obtained. This effect by the binaural sound recording is achieved as a consequence of an audio signal having an acoustic characteristic extremely close to that of a sound arriving at a human eardrum, which audio signal is obtained by recording a sound near both ears, in particular, near an entrance of an external auditory canal.
Until a sound arrives at the human eardrum from a sound source (e.g., a speaker and a musical instrument), the timbre thereof is changed because of reflection and diffraction caused at various objects. Among these objects, the influence of a head, a torso, and an auricle of a listener gives a human a clue for perceiving a direction of arrival. Information representing a transfer characteristic of a sound from a sound source to eardrums of both ears within a free space is called a head-related transfer function (HRTF). Once a database is compiled on the head-related transfer function, a sound can be expressed three-dimensionally by being played back with headphones through signal processing based on the head-related transfer function without putting the binaural microphones on both the ears of a user every time for recording. However, the binaural microphone is required also to measure the head-related transfer function.
Typically, when a researcher or the like measures the head-related transfer function, an impression material is inserted to the inside of the external auditory canal and a microphone is fixed at a position where a sound receiving surface thereof is visible from an external auditory canal entrance. There is a first advantage of a method of this measurement method in that, because the binaural microphone can be created so as to fit into a personal shape of the external auditory canal, the microphone is always fixed at the same position and thus, a position shift of the microphone over time and a position shift of the microphone before and after removing and putting the microphone are unlikely to occur. Particularly, in order to accurately replicate the resonance at a cavum conchae within the auricle, portions of the impression material and the microphone protruding to the cavum conchae from the external auditory canal are required to be kept minimized (for example, refer to Non-patent Document 1). When the cavum conchae is filled with the impression material or the microphone, the resonance generated at the cavum conchae is hindered, causing a possibility of accurate localization, natural localization, and a timbre not being able to be reproduced. When the impression material is used to set up the binaural microphone, by observing these precautions depending on a personal shape of the external auditory canal, a binaural microphone having reproducibility can be obtained.
As another advantage of fixing the microphone at the inside of the external auditory canal by using the impression material to measure the head-related transfer function, a fact that an external sound is blocked from entering the inside of the external auditory canal and accordingly, the resonance at the inside of the external auditory canal is not generated is given. There are large differences in the resonance at the inside of the external auditory canal from person to person and thus, a possibility of an adverse influence occurring in the localization when a person other than a measurement subject hears is increased. Such a sound recording approach is called a “block approach”, which is a method generally used in the measurement of the head-related transfer function.
In the above-described method, however, every time a wearer who wears the microphone during recording is changed, it is necessarily required to fill the external auditory canal of the wearer with the impression material. This work causes unnecessary damage to the microphone and also acts as a load for the wearer and a worker, while bringing about a safety problem on the ears of the wearer.
Meanwhile, a canal type earphone device used while being inserted deeper into an earhole than the case of an inner ear type has been widely known. For example, an earphone device has been proposed in which, by arranging a sound conduit diagonally from a position off the center of a housing, the housing is accommodated in the cavum conchae while the sound conduit is arranged up to the external auditory canal entrance (for example, refer to Patent Document 1). In this type of the earphone device, however, the sound conduit only can be inserted until a point just before a first curve of the external auditory canal. In addition, an ear piece is inserted into a root portion of a relatively soft tragus and thus, a sufficient sealing effect cannot be obtained.